Large business enterprises nearly always employ multiple data centers since customer, supply chain, and internal user response requirements make it most efficient to store on-line data close to where it is used. Inevitably, this means that the same data must be available at several widely separated locations. Price lists, product specifications, web pages, and similar data must often be replicated at several of the enterprise's operating locations.
It is important that such data be consistent throughout the enterprise. If data is to be consistent, it must be replicated to remote locations where it is used. Changes to all data replicas must be synchronized so that they appear at all locations at approximately the same time. Perhaps one of the most important reasons for data replication is disaster recovery. Enterprises need strategies for recovering their ability to process data soon after an event, such as a software failure or a hardware failure, incapacitates an entire data center or the data in it. An up-to-date replica of the operational data at a remote location unaffected by the failure causing event can mean the difference between rapid recovery and total enterprise failure.
Ideally, mirroring data would fulfill the business needs met by data replication technology. Two facts relating to distributed data centers make mirroring inadequate. More particularly, distances between primary and secondary data centers, routing hops, and momentary network overloads can make data update transmission time too long in terms of its affect on application response. Additionally, brief network outages can occur frequently enough to make the frequent mirror resynchronization they would imply unacceptable.
However, data transfer technology has advanced so that the distances between primary and secondary data centers having mirrored data, can be stretched without concerns of momentary network overloads. For instance, data centers can be separated by up to 100 kilometers using dense wave division multiplexing in the data link therebetween. Unfortunately, failures of the data link between data centers still require mirror resynchronization.
Mirror resynchronization can be a lengthy process that oftentimes requires a full resynchronization in which data is copied from one minor to the other in a block by block process until all data blocks of the minor have been copied.